1. Field of the Invention
The present invention generally relates to increasing the depth of field in laser scanning heads of systems for electro-optically reading indicia, especially bar code symbols.
2. Description of the Prior Art
Many industries, particularly the grocery and food processing industry, have begun to designate their products with a unique bar code symbol consisting of a series of lines and spaces of varying widths. Various bar code readers and laser scanning systems have been developed to decode the symbol pattern to a multiple digit representation for check-out and inventory purposes.
Aside from the conventional contact-type and non-contact-type wand or pen bar code readers, true laser scanners, such as point-of-sale or deck scanners of the moving-beam or fixed-beam type, have been built into stationary counters. However, these deck scanners are large, massive, stationary installations. Some symbol-bearing objects are too heavy, or too big, or too inconvenient to be brought to the stationary scanning installation. Some objects may be stationary themselves.
In order to provide a mobile scanning system, semi-portable laser scanning heads, such as disclosed in U.S. Pat. No. 4,251,798, were designed to permit the user to bring the scanning head to the object. However, such semi-portable heads weighed over three pounds, had to be partially supported by the object itself, and were too heavy and somewhat difficult to readily manipulate, particularly for those assembly-line applications where a user was routinely required to repetitively manipulate the semi-portable head many times per minute, every hour and on a daily basis.
More modern miniature laser scanners weighing on the order of two and one-half pounds, such as described in U.S. application Ser. No. 125,768, filed Feb. 29, 1980, entitled "Portable Laser Scanning System and Scanning Methods," and assigned to the same assignee as the present application, have recently been proposed to provide a more truly portable laser scanning head which is supportable entirely by the user during the reading of the bar code symbols. Although generally satisfactory for its intended purpose, this fully portable head had a rather large case width due to the fact that the laser beam generated within the head was required to be swept over a wide field of view across the bar code symbol, which is located in the vicinity of a reference place located exteriorly of the housing. The swept laser beam, which was accommodated entirely in the wide-bodied head, therefore, required a great deal of "dead" space within the head. This "dead" space caused the case width to be unnecessarily large, and undesirably added to the overall size and weight of the head, thereby detracting somewhat from its features of ease of manipulation and full portability.
Another drawback of the wide-bodied head was that the case width increased from the rear towards the front of the head, as considered in the direction from the housing towards the reference plane, with the result that the front or nose of the head had a wide snout. In a preferred embodiment, the wide-bodied head had a gun-shaped configuration. It was desirable in some applications to insert the gun-shaped head, when not in use, into a user-supported holster of the kind traditionally used to receive and support firearms. However, the wide snout on the wide-bodied head did not lend itself to conveniently fit in the traditional gun holster. Hence, the wide-bodied heads were typically tossed and left on the nearest table, and were prone to being damaged, misplaced and lost.
Still another drawback associated with the semi-portable and wide-bodied heads was that any dirt specks on the exit window through which the laser beam exited en route to the bar code symbol, affected the laser scan at the symbol. The greater the distance between the exit window and the symbol at the reference plane, the less of a potential malfunction would result from dirt specks on the exit window. However, the known semi-portable and wide-bodied heads were too dirt-sensitive for some applications inasmuch as the exit window was too undesirably close to the reference plane.